The hypothalamic hamartoma (HH) is a rare congenital human brain malformation associated with gelastic (or laughing) seizures that are difficult to diagnosis early in life and are notoriously refractory to medical therapy. Despite increasing clinical recognition of this condition, the mechanisms of epileptogenesis are largely unknown. Importantly, seizures have previously been shown to originate from the hamartoma itself. At the Barrow Neurological Institute, we have treated over 100 patients with this condition over the past several years, constituting the largest single-center series ever;we are currently the only multidisciplinary program for HH management in the United States, and the most active HH program in the world. Thus, we are uniquely positioned to study the mechanisms of epileptogenesis in HH. The major goal of the proposed studies is to determine the mechanisms of seizure genesis in surgically resected HH tissue using a combination of cellular electrophysiological, fluorescence imaging, immunocytochemical and gene expression techniques. Our preliminary data demonstrate that HH tissue contains two distinct populations of neurons;small HH neurons exhibit intrinsic pacemaker-like activity, whereas large HH neurons are quiescent. We have demonstrated that a subpopulation of large HH neurons depolarize when perfused with the GABAA-receptor agonist muscimol. Based on these intriguing findings, we hypothesize that such GABAA-receptor mediated depolarization may be a consequence of decreased expression of the cation-chloride co-transporter KCC2 (relative to its cousin, NKCC1), and that such responses may contribute to seizure genesis. Specifically, we hypothesize that muscimol will increase intracellular calcium levels, an effect that can be blocked by the NKCC1 antagonist bumetanide. Further, we hypothesize that HH tissue will demonstrate decreased expression of KCC2 relative to NKCC1 using immunocytochemical and single-cell RT-PCR expression techniques. Although HH itself is uncommon with an estimated prevalence of 1 in 50,000-100,000 it is perhaps the best human model for subcortical epilepsy, and one of the most intriguing models for understanding the consequences of catastrophic epilepsy in childhood. As such, a detailed scientific understanding of this disorder has major ramifications for expediting translational research focused on the developmental epilepsies. PUBLIC HEALTH RELEVANCE: Epilepsy is a common neurological disorder that afflicts more than 3 million people in the United States alone, and many newly diagnosed patients are infants and children. There is a substantial need to understand how epilepsy develops in the pediatric population in order to develop newer, more effective treatments. This research proposal, while focused on a rare brain tumor that causes uncontrolled seizures, may provide novel insights into the mechanisms of seizure propagation and progression.